Patent document DE 10-2011-08657-2-A1 and corresponding US 2013/0251540-A1, PAULUS & SCHAAB/KSB, published 26 Sep. 2013, disclose a method and a control device for RPM-variable control of a displacement pump apparatus. The displacement pump apparatus includes a displacement pump and a drive, consisting of an electrical drive motor and a frequency converter. A final or output pressure of the displacement pump is, via a control device, adjusted to a target or setpoint value. The control device makes adjustments, based upon instantaneous values provided by the drive. Thereby, the control device regulates to the target or setpoint value.